If the female hops toward him, or at least if she does not hop away, the male begins what the Grant team calls the “sex chase,” flying after her on a “twisting and undulating” flight path. He swoops and pivots in mid-air in what looks to the finch watchers like maneuvers to keep the female on his territory. The female stays just a wing-beat ahead of him making high-pitched cries, “Kew, kew, kew!”
These are ground finches, remember. Their wings are short and stubby; they are not really cut out for this kind of aerial acrobatics. The long twisting flights must require fantastic amounts of energy to keep up, especially after a long dry season of thirst and famine. In other words, the sex chase is expensive. But all the finches do it when they court, young and old alike, even birds that have mated together for years.
Laurene Ratcliffe and Peter Grant watched more than a thousand finches pair off and go chasing each other through the air. In all those courtships, they saw a male go after a female of the wrong species only twenty-six times. And in the middle of each of those rare cases, either the male broke off the courtship or the female flew away. In four of the mismatched courtships, the male stopped flirting and actually attacked the female.
Long ago, before Lack, an ornithologist speculated that Darwin’s finches might all be one intermingling species, a hybrid swarm: finches just mated with finches. That speculation (Peter Grant calls it a “cry of desperation”) could only have come from a scientist who had studied the finches in the museum trays and not out on the islands.
Since the very first year of their watch, the Grants have known that Darwin’s finches have a talent for telling one another apart. The birds’ ability to recognize one another is impressive, even a bit mysterious, given the remarkable range of variation in their beaks and bodies and the drab sameness of their plumage. It is true that the six species of ground finches look different from the six species of tree finches. The tree finches’ plumage is a yellowish-green, the general tint of Galápagos trees in leaf, and the ground finches’ plumage is black and mottled brown, the general tint of Galápagos lava. They are markedly different birds, even though the tree finches do spend much of their time on the ground and the ground finches spend much of their time in trees.
But there is no obvious difference in plumage among the six species of ground finches. There is no obvious difference in their styles of courtship either; the singing post, the flirtations at the display nest, the sex chase—all seem to be the same from species to species. There is no great difference in their territories either. On Daphne Major in good years a map of the male finches’ turf is a sprawling jumble of intersecting circles and oblongs. A male finch won’t let another male finch near its nest, so the epicenters of these territories, their capitals, so to speak, are always separate. And a male scandens will not let another male scandens claim his territory, so their turf is always separate. But the territories of scandens will overlap with the territories of fortis. On a map all these territorial borders look like the ripples of raindrops in a pond. In the middle of this overlapping confusion, every fortis has to find a fortis; every scandens has to find a scandens.
The advertising song that each male broadcasts from his singing post, or from his nest, is distinctive enough that the Grants and the other finch watchers learn to recognize some individuals by their songs. And this is something birds can do too. Male hooded warblers can tell their neighbors apart by their songs during the breeding season. Even after an eight-month hiatus, during which the warblers stop singing, migrate to Central America, and then return to their breeding territories, the males still remember and recognize their rivals’ songs.
Among Darwin’s finches there are usually not one but two or more versions of the advertising song in circulation within each species, on each island. Some males sing one version, and some males sing another. On Daphne and Genovesa, a very few birds sing more than one. But the average male has a very limited repertoire. He has just one advertisement for himself, which he sings over and over all his life.
The Grants have spent many hours listening to the songs in the islands, and studying the song-graphs at Princeton. In the graph, the song of one of Daphne’s champion breeders, Number 2666, is a sort of ghostly pyramid of six gray, fine-lined blotches. Finches Number 2663, 2664, and 2665, the siblings of 2666, sang exactly the same song, and so did their father. In fact most males sing the song of their father. The Grants have recorded the song of 2666 year after year, and the song-graphs are all the same, they don’t change. “Peter says it sounds like ‘Mostly Muesli,’ says Rosemary, and laughs. “It goes Mostly Muesli, Mostly Muesli, Mostly Muesli.…’ ”
Another advertisement song on Daphne is more of a chuh-chuh-chuh, she says. Scandens is mainly chuh-chuh-chuh-chuh-chuh. Rosemary can sing them all aloud as she leafs through the song-graphs. “Chae-ae-ae. This one is wicki-picki, wicki-picki, wicki-picki, very fast.”
Besides their advertising songs, all of the finches can also produce a sort of hissing whistle, a very high, descending note. All of the ground finches’ whistles sound much alike (although on the island of Pinta the sharp beaks add a distinctive, drawn-out buzzz-clink!). So it is the advertising song that the Grants have studied most closely as a clue to how it keeps the species apart.
Laurene Ratcliffe has discovered subtle variations even in songs that sound alike in the islands and look much alike in the song-graphs. These variations are picked up and passed down from father to son. In a series of experiments, she and Peter Grant have taken tapes of some of these songs and broadcast them from a speaker in the center of a finch’s territory. More than nine out of ten males reacted strongly and aggressively to the broadcasts. They seem to respond equally to either of their own species’s song types, A or B. That is, whether they themselves sing song A or song B, they will fly over the speaker, countersing with it, land on it or stand in front of it, and stare at it from close range. They react as if to drive away a strange male who has set up shop and begun to sing in the middle of their territory. Even on islands where two species sing much the same song, a male will respond much more vigorously when it hears one of the songs of its own species coming out of the loudspeaker than when it hears a song of the other species. Laurene and Peter tried these kinds of tests with several different pairs of species of ground finches on several islands, always with the same results.
Singing the right song is important; a finch that learns the wrong song is in trouble. The finch watchers have seen, year after year, a cactus finch singing away from his tallest cactus in the prime of health. Year after year he sings the song of fortis. He is not getting any younger, a good bird and a good singer doomed to a genetic death.
Apparently, every once in a while, instead of learning the song of his own father, a male finch learns the song of a neighbor; once in a blue moon, he learns the song of a neighbor of the wrong species. None of the finch watchers has ever seen this kind of accident in progress, only the result, full-grown ground finches singing the wrong song. Perhaps, Peter Grant writes, “they lost contact, as fledglings, with their fathers, were fed by a male of another species, and became misimprinted on his song.” They may even have acquired the strange song in the nest, Peter believes. There are opportunities. Dolph Schluter says he once had a nest on Pinta in which a female sharp beak and a female small beak each laid three eggs. All six eggs hatched, and both pairs of parents, the sharp beaks and the small beaks, squabbled over ownership of the nest until the sharp beaks won out. Unfortunately the six chicks were eaten by a hawk before they ever got out of the nest.
Laurene Ratcliffe once spotted a small beak, a male, on Española, that was keeping two houses a few meters apart. In one cactus he was nesting with a female of his own species, and in the other he was feeding the nestlings of a warbler finch.
(“This is a big controversy in Scandinavia right now,” says Trevor Price. “A male has two nests. Does she know? That’s all they talk about in Scandinavia. And it looks like she does know, but that’s the best s
he can do.”)
“Misimprinted birds are interesting because they broadcast not one identity but two,” writes Peter Grant: “a false one at long distance and a true one at short distance.” This is one way the finches can get their lines crossed and make hybrids. The misimprinted males tend to get into fights with males that sing their song—that is, with the wrong species—and they tend to breed with the wrong species too, when they mate at all.
EVEN THOUGH HIS THINKING about evolution was formed by what he had seen in the Galápagos, Darwin never quite understood how much species are like islands. The momentum of his long argument sometimes made him sweep past and ignore the distinctiveness of species, and talk as if all the categories of life run smoothly together. “I look at the term species as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other,” Darwin writes in the Origin, as if species were just as arbitrary a convention as varieties, which are merely “less distinct and more fluctuating forms.”
It is true that the thirteen species of Galápagos finches are things of the passing moment, like the bodies of the individual birds that make up each species, like the islands that are their home, and like the very planet that is the home of all these islands. But one of the most significant facts about bodies, islands, and planets is that while they last, they are real, distinct, and separate. True species are as real as bodies, islands, or planets, though what holds them apart is not as uniform or as obvious to the eye. Even now, even with hybrids flourishing triumphantly on Daphne Major, most of the finches on the island seldom interbreed.
Clearly song is part of the finches’ secret. But song is not everything. If it were, the cactus finch that sings a fortis song would be able to mate with a fortis female, and the broadcasts of the loudspeakers that Laurene and Peter arranged on Daphne and other islands would have had an effect on both male and female finches. The males often attacked the speakers, but the females acted completely uninterested. In almost five hundred trials, only three females ever came over to the speakers.
To find out what was missing for the females, Peter and Laurene set up two speakers 10 meters apart on the lava and placed a stuffed male decoy on top of each one. Now the females were more interested. They hopped over to investigate each speaker. They came closer to a male decoy of their own species than a male of another, and spent more time near it. They also spent more time with a male from their island than a male from another.
Obviously the females are looking for something more than song. So are males, since when females approach a singing male, the females do not sing, yet the male decides somehow whether the female is one of his own species, a bird of his feather, worth the trouble and expense of courting.
The word species comes from the Latin verb specere, to see. Linnaeus used the word to mean groups of animals and plants that look distinctly different to the eye. After screening each other with songs, Darwin’s finches must do their taxonomy as Linnaeus did. They must tell one another apart by eye.
To find out what the birds are looking for, Laurene Ratcliffe and Peter Grant performed a slightly ghoulish series of experiments. They collected dead finches from the lava, and also a few stuffed specimens from the museum at the Charles Darwin Research Station. They spruced up each corpse a bit and rejuvenated it with a spine of wire, so that they could bend the body into various poses. They also painted the beaks of the females dark brown and the beaks of the males black: mating colors.
They went to the heart of a male finch’s territory, right near his nest. There they propped a female scandens decoy on one cactus pad and a female fortis on another cactus pad. They tried perching the female decoy in various poses, sometimes in sitting position and sometimes in the much more provocative pose in which the beak is angled upward toward the sky, the wings extended a bit, and the tail spread, an invitation to copulation. In the body language of the finches this is a powerful invitation. In fact when Ratcliffe and Grant first bent a model into this shape, male finches began courting it and trying to mount it while the stuffed bird was still in their hands. On Pinta, where the nests were quite high, Laurene would lash the decoys to a bamboo pole and then prop the pole up at the nest. When she carried the pole from territory to territory, males would swoop onto the decoys and try to mate with them. Laurene learned to cover the decoys with a bandanna until she had them ready.
Each experiment began when they took away the bandannas, unveiled the two rival female decoys, and backed away. Within minutes a male would come near and inspect the models. Usually, Ratcliffe and Grant write, the male faced the decoy, “wing-quivering and whistling, and occasionally swaying from side to side.” Then, within five or ten seconds, the male mounted and copulated with the decoy. Afterward, while still mounted, he sometimes touched his beak gently to the females head or pecked it gently.
The finch watchers tested more than one species, and they repeated the tests on more than one island. Male finches most often chose the decoy of their own species. They did also show some sexual interest in females of alien species, although Grant and Ratcliffe noticed that very often in those cases the males’ displays “suddenly collapsed in mid-performance.”
Finches courting. He shakes his wings; she raises her beak into the air, a sign of interest.
Drawing by Thalia Grant
Finally, in the most interesting and grotesque refinement of these experiments, Ratcliffe and Grant tried switching the heads and bodies of fortis and scandens on the decoys. They found that they could produce a smooth gradation of responses. A decoy with the head of a scandens and the body of a scandens got the most reaction from a scandens. A decoy with the head of a scandens and the body of a fortis got a smaller response. A decoy with the head and body of a fortis got the least response. Of course the males may have been put off by these weird-looking decoys. But if they were, there was no sign of it: their responses to the intermediate females tended to be more or less intermediate. They were not just going through the motions either: they left drops of semen on the decoys’ tail feathers.
Many other species of birds have been shown to be able to tell one another apart on sight. But most of them seem to go by bold differences in markings and plumage: a red beak, a black head, a scarlet breast, Day-Glo yellow eye-rings. Not many birds rely on differences in size and shape as subtle as the cues that Darwin’s finches are using.
The Grants would like to carry out more elaborate experiments, but the regulations of the National Park Service of Ecuador, and the Grants’ own feeling for the birds, forbid them. Based on what they have seen in the wild, the species are kept apart by two barriers, one at a distance and one close up, like a moat and a wall. The first hurdle is song. The second is the close-up visual inspection. And here the cadaverous experiments show that something about the heads of the birds is crucial.
Now, there is nothing distinctive in the sizes, shapes, or feathers of the finches’ heads. There is only one character above the neck that varies distinctively from species to species. The conclusion is inescapable: the feature that makes the finches most interesting to us is also the feature that makes them most interesting to each other. When they are courting, head to head, making decisions that are fateful for the evolution of their lines, Darwin’s finches are studying the same thing as the finch watchers. They are looking at each other’s beaks.
UNLIKE COASTLINES OF COLD lava, instincts are malleable. Natural selection can shape sexual preferences. So mating patterns can shift, change, evolve, like the beak of the finch.
Experiments in artificial selection have created spectacular and sometimes rather monstrous transmutations of sexual instincts. In one lab, investigators raised fruit flies of the species Drosophila subobscura in total darkness. After fourteen generations, the males no longer gave the typical courtship dance. Instead they tapped around in the dark until they found a female and then tried to force copulation; the females did not turn them away. “It takes just fourteen generations to turn a Drosop
hila subobscura from a courtly dancer to a blind, tapping rapist,” notes the student of evolution James Shreeve. “How perishable are our essences! No doubt Aristotle would find all this fiddling around with the edges between things to be very dark stuff indeed.”
The origin of species is the origin of the invisible, isolating walls that arise between two populations and make them living islands. Individuals vary in the characters that affect their attractiveness at mating time, just as they vary in characters that affect their capabilities to feed or fly. Variations in these characters can lead to sexual isolation, as with the poor bird that sang the wrong song. Sometimes variation can grow broader, producing what have been called “sex races.” Gypsy moths in Japan and Korea are divided into races like these. Cross a male from one race and a female from another, and you may get females with the bodies of males, or whole cohorts of moths that look androgynous and are completely sterile. The sex races shift from one to the next, and to the next, as you travel south and west from Hokkaido to Honshu to Kyushu. Here we seem to see invisible barriers forming that can carve one species into many.
The drosophilist Ken Kaneshiro has made a special study of the courtship patterns (“often bizarre”) of Hawaiian Drosophila. Kaneshiro calls these flies “the birds of paradise of the insect world.” Like Darwin’s finches they make natural subjects for the study of evolution in action. There are more than seven hundred species on the islands, and more new species are being collected all the time. The Hawaiian Drosophila Project is one of the most comprehensive efforts in the history of zoological classification.
Kaneshiro argues that sexual selection is “a powerful force in initiating the speciation process.” Often the chromosomes of two species of Hawaiian fruit flies look almost identical, yet their genitalia are strikingly different. Kaneshiro and his colleagues are now looking more closely at the genes at the molecular level to try to find the genetic changes that make the difference. Sexual selection seems to drive these divergences, Kaneshiro argues, because “all the sometimes bizarre secondary sexual characters found in the males are used in some way during their complex sexual displays.”
The Beak of the Finch: A Story of Evolution in Our Time Page 20
